1. Field of the Invention
The present invention relates to autoranging, and, in particular, to passive methods and apparatuses for determining the range of one or more objects and/or for focussing a camera.
2. Description of the Related Art
In the field of package shipping, it is desirable to have a camera positioned overhead a conveyor system to read package labels as packages are propelled along the conveyor system. Since package heights may vary greatly, the field of view of the camera may also vary greatly. It is therefore desirable to use a high-resolution camera (e.g., with a 4096-pixel linear array charge coupled device (CCD) sensor) to generate images of the package labels. The camera system preferably provides autofocussing of the high-resolution camera to adjust for the varying package heights.
One known technique for focussing an overhead camera employs a light curtain consisting of a one-dimensional vertical array of light-emitting diodes (LED) and a matching one-dimensional vertical array of photo-transistors. The LED array is positioned on one side of the conveyor to transmit radiation across the conveyor path toward the photo-transistor array positioned on the other side of the conveyor. Package height is indicated by the lowest non-interrupted light beam as the package passes between the LED and photo-transistor arrays. This height estimate is then used to focus the high-resolution overhead camera.
This technique of autoranging can only measure the highest point of a stack of packages. Moreover, the resolution is limited by the density of LED/photo-transistor pairs in the matched arrays.
This technique of autoranging also employs active autofocussing, wherein radiation is generated with the specific purpose of focussing. Active autofocussing contrasts with passive autofocussing in which only ambient radiation is required to perform the focussing function.
Another known technique for determining package height employs scanned-laser triangulation. In this technique, a laser spot is scanned across the package surface by means of either a gavanometer stepped mirror or by a rotating mirror polygon. The range of the package is inversely proportional to the displacement of the reflected spot as imaged by either a line-scan CCD array or by a lateral-effect photodiode (i.e., a position sensing device).
This technique takes considerable time to scan mechanically the laser spot and limits the applicability of this technique to slow moving conveyor systems. In addition, there is the risk of injury to people from the laser radiation.
Yet another known technique for determining package height employs projected-pattern triangulation. In this technique, either a single light strip or a pattern of light strips are projected onto a package surface. Displacements of the projected light pattern imaged by a two-dimensional CCD array are inversely proportional to the range of the package.
Under this technique, the projected light pattern may be difficult to see in the presence of intense ambient light used for the high-resolution camera. Moreover, it takes considerable time to acquire a two-dimensional image.
It is also desirable, in the field of package shipping, to determine the heights of stacks of nested packages. Robots are preferably used to place and remove packages from nested stacks for storage and/or shipping. To function efficiently and automatically, the robots preferably have information regarding the height of each package stack.
Subbarao (U.S. Pat. No. 4,965,840) discloses the use of a pair of cameras with differing parameters and characteristics to image a scene for the purposes of either object range determination or autofocussing. The system of Subbarao estimates blur spread, which may then be used to determine object range and/or generate autofocus parameters. However, the system of Subbarao employs an algorithm that requires knowledge of the camera parameters for the pair of cameras, such as the aperture size, the focal length, and the distance between the second principal plane and the image detector plane.
It is accordingly an object of the present invention to overcome the disadvantages and drawbacks of the prior art and to provide an improved passive system for range determination and/or camera focussing.
It is a further object of the present invention to provide a system for passive autoranging without requiring any knowledge of camera parameters such as aperture size, focal length, or the distance between the second principal plane and the image detector plane.
It is a further object of the present invention to focus a high-resolution camera positioned overhead a conveyor system in order to generate images to read labels on packages on the conveyor system.
It is a further object of the present invention to determine the heights of a plurality of stacks of nested packages.
It is a further object of the present invention to stabilize the blur-spread estimates to random variations of the data (i.e., the perceived images). Stabilized blur-spread estimates imply more consistent and more accurate range and focus setting calculations.
Further objects and advantages of this invention will become apparent from the detailed description of a preferred embodiment which follows.